X-ray systems are used for imaging in support of diagnostic examinations and for interventional procedures e.g. in cardiology, radiology and surgery. Said X-ray systems consist of at least one X-ray tube, an X-ray detector, corresponding mechanisms, a high-voltage generator, an imaging system, a control unit, and where appropriate a patient couch. Conventional X-ray systems employ integrating X-ray detectors which utilize either scintillators or amorphous semiconductor layers such as selenium for X-ray conversion.
In order to enable new clinical applications such as spectral imaging, but also to achieve a further gain in quantum efficiency, investigations are increasingly focusing on the potential of counting detectors or energy-discriminating counting detectors based on direct-converting materials, such as CdTe, CZT (CdZnTe), Si or GaAs, and contacted ASICs.
A counting digital X-ray image detector for acquiring X-ray images of an object irradiated by X-ray radiation has at least one detector module. The detector module comprises a planar direct converter for converting X-ray radiation into an electrical signal and an array composed of a multiplicity of counting pixel elements, wherein each counting pixel element has a charge or signal input, a conversion device for converting the electrical signal into a count signal, a digital counter for capturing and storing the count signal, and a driver and readout unit. The X-ray image detector is embodied in such a way that each pixel element of the X-ray image detector is connected to the corresponding electrodes of the detector material of the direct converter by way of contacts (bump bonds).
The digital counting X-ray image detector is able to determine the number and/or energy of the events associated with each pixel element or each pixel cluster. The digital counting X-ray image detector provides spatially resolved information relating to the number and/or energy of the events and in addition it offers the possibility of energy-selective imaging.
A pixel element constitutes the smallest detection entity having a connection to evaluation electronics circuitry in the ASIC of the X-ray image detector. A plurality of discriminators are possible per pixel element.
A pixel cluster constitutes an entity composed of a plurality of pixel elements with or without common evaluation logic. The pixel cluster allows a more complex evaluation and/or an evaluation as a function of the neighbor pixel elements; a plurality of discriminators are possible per pixel cluster.
The digital counting X-ray image detector can provide a number of counting modes.
Various effects can now lead to a situation in which an absorbed X-ray quantum deposits its energy not only in one pixel element, but that due to processes such as charge sharing or fluorescence photons (k-fluorescence) some of the energy is deposited in the neighboring pixel elements or is not taken into account. This can lead to miscounts and/or incorrect assignment of the energy in the case of energy-discriminating detectors. Summation and anticoincidence circuits can be utilized in order to mitigate or avoid these effects.
A counting digital X-ray image detector for acquiring X-ray images of an object irradiated by X-ray radiation is known from the as yet unpublished German application bearing the official application number 102013219740.3, the entire contents of which are hereby incorporated herein by reference.
At high X-ray fluxes it becomes more difficult to discriminate independent events and the probability of miscounts increases. In the evaluation of signals as a function of the neighbor pixels, a number of pixel elements are blocked, thereby further increasing the proportion of miscounts. The aim of good energy resolution is at odds with a maximally error-free count at high X-ray fluxes.